1. Field of the Invention
Aspects of the present invention relate to a structure of a bipolar plate used for a fuel cell.
2. Description of the Related Art
A fuel cell is an electrical generation system that transforms chemical energy directly into electrical energy through a chemical reaction between hydrogen that is contained in a hydrocarbon group material, such as methanol, ethanol, or natural gas, and oxygen.
A proton exchange membrane fuel cell (PEMFC) has advantages of superior output, low operating temperature, rapid starting, and speedy response time compared to other fuel cells, and is the preferred fuel cell for automotive, portable, residential and small commercial applications. An example of a proton exchange membrane fuel cell is a direct liquid feed fuel cell.
FIG. 1 is a cross-sectional view of a basic configuration of a conventional PEMFC, specifically, a direct liquid feed fuel cell. As depicted in FIG. 1, a conventional PEMFC has a structure that includes an anode electrode 2, a cathode electrode 3, and an electrolyte membrane 1 interposed between the two electrodes 2 and 3. The anode electrode 2 and the cathode electrode 3 respectively include diffusion layers 22 and 32 that supply and diffuse a fuel, catalyst layers 21 and 31 at which oxidation and reduction reactions of the fuel occur, and electrode supporting layers 23 and 33. A theoretical voltage output from a unit cell of a direct methanol fuel cell (DMFC) is approximately 1.2 V. However, an open circuit voltage at ambient temperature and atmospheric pressure falls below 1 V due to a voltage drop caused by an active surcharge and a resistance surcharge. In practice, an actual operating voltage of the unit cell lies in the range of 0.4˜0.7 V. Therefore, to obtain higher voltages, a plurality of unit cells connected in series is required.
A fuel cell stack is formed by stacking a plurality of unit fuel cells that are electrically connected in series with each other. A conductive bipolar plate 4 is interposed between adjacent unit cells to electrically connect the unit cells to each other.
The bipolar plate 4 may be formed, for example, of a graphite block having high mechanical strength, high electrical conductivity, and good workability. A block of a composite material containing a metal or a conductive polymer can also be used as the bipolar plate 4. Flow channel 41 and flow channel 42, which independently supply fuel and oxidant (typically, air) to an anode 2 and a cathode 3 contacting the bipolar plate 4 are formed on respective surfaces of the bipolar plate 4. In other words, the bipolar plate 4, when placed in the fuel stack, has one surface that faces an anode 2 of a unit cell and includes flow channel 41 and has an opposite surface that faces the cathode 3 of another unit cell and includes the flow channel 42. On an uppermost and a lowermost end of the fuel stack, end plates (not shown), which are monopolar plates that respectively supply fuel or air to the anode electrode 2 or the cathode electrode 3, are disposed. The end plates respectively include the flow channel 41 or the flow channel 42 (see FIG. 1) for supplying fuel or air to the contacting unit cells.
FIG. 2 is a plan view of a surface of a conventional bipolar plate 4 for a conventional PEMFC In particular, FIG. 2 shows a surface where flow channels for a cathode are formed.
Referring to FIG. 2, in the conventional bipolar plate 4, a plurality of flow channels 42, of which upper parts thereof are opened, are formed in an electrode region 47 where a membrane electrode assembly (MEA) is disposed. Between the channels are lands 48 that contact the MEA. A region outside of the electrode region 47 includes manifolds 46 and 46′ connected to an inlet or an outlet, respectively, of the flow channels 41 and fuel path holes 43a, 43b, 44a, and 44b that are through holes for supplying or discharging hydrogen fuel or oxidant by connecting to the manifolds 46 and 46′ and that perforate the bipolar plate 4. The fuel path holes 43a, 43b, 44a, and 44b constitute an inlet 43a and an outlet 43b of the hydrogen fuel and an inlet 44a and an outlet 44b of the oxidant.
The flow channels 42 in FIG. 2 can be formed to have a simple structure in which the flow channels have the same cross-sectional area (same width and same depth). In the conventional bipolar plate 4 having the flow channels 42, the concentration of oxygen and hydrogen in a gas (air or a reformed hydrogen gas) that flows in the flow channels 42 is reduced as it flows. Accordingly, the current density of the fuel cell is not uniform, and reaction heat may be locally increased. Also, oxygen concentration (or hydrogen concentration at the anode electrode) at a portion of a vertical cross-section of the flow channels 42 that contacts the membrane 1 (see FIG. 1) is lower than the concentration of oxygen at a bottom portion of the flow channels 41, thereby reducing the efficiency of the fuel cell.